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MCB tripping - why?



 
 
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  #1  
Old December 29th 09, 11:41 AM posted to uk.d-i-y
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Posts: 117
Default MCB tripping - why?

Hi, the MCB on my split circuit consumer unit is tripping
sporadically, sometimes when nothing is connected to the downstairs
socket circuit. The MCB protects the RCD protected circuits (upstairs
and downstairs sockets, cooker, hot tank, kitchen sockets). The hot
tank and cooker circuits are turned off at the rcd and are not
connected used/connected yet.The light (non-RCD'd) circuits stay on
when the RCD trips.

Please can someone tell me what causes an MCB to trip, other than
overcurrent, and not trip an RCD, and hopefully some possible things
to check in order to locate the fault.

This 'may be' heating associated but does happen when the heating is
off and the boiler is off i.e. could it be associated with hot pipes
near a cable?

Many thanks.
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  #2  
Old December 29th 09, 02:16 PM posted to uk.d-i-y
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Posts: 888
Default MCB tripping - why?

On Tue, 29 Dec 2009 02:41:08 -0800, nafuk wrote:

Hi, the MCB on my split circuit consumer unit is tripping sporadically,
sometimes when nothing is connected to the downstairs socket circuit.
The MCB protects the RCD protected circuits (upstairs and downstairs
sockets, cooker, hot tank, kitchen sockets). The hot tank and cooker
circuits are turned off at the rcd and are not connected used/connected
yet.The light (non-RCD'd) circuits stay on when the RCD trips.


Some confusion he one generally has an RCD giving residual-current
protection to one or more MCB-protected circuits, not the other way round.
Please can you confirm your layout of MCBs and RCDs?

(The RCD is the one with the 'TEST' button.)

Please can someone tell me what causes an MCB to trip, other than
overcurrent, and not trip an RCD, and hopefully some possible things to
check in order to locate the fault.


Overcurrent is the only thing that causes an MCB to trip. A difference in
current between live and neutral causes an RCD to trip. Thus a fault that
causes excess current to flow (slightly excess for a long period or gross
excess for a short period) trips an MCB (or other over-current device
such as a fuse). A few tens of milliamps leakage to earth trips an RCD -
even touching neutral and earth together on a circuit switched off at the
MCB or other single-pole switch in the circuit can cause enough earth-
loop current to flow to do it.


--
John Stumbles -- http://yaph.co.uk

Xenophobia? Sounds a bit foreign to me.
  #3  
Old December 29th 09, 03:05 PM posted to uk.d-i-y
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Posts: 1,009
Default MCB tripping - why?



"John Rumm" wrote in message o.uk...
nafuk wrote:
Hi, the MCB on my split circuit consumer unit is tripping
sporadically, sometimes when nothing is connected to the downstairs
socket circuit. The MCB protects the RCD protected circuits (upstairs
and downstairs sockets, cooker, hot tank, kitchen sockets). The hot
tank and cooker circuits are turned off at the rcd and are not
connected used/connected yet.The light (non-RCD'd) circuits stay on
when the RCD trips.

Please can someone tell me what causes an MCB to trip, other than
overcurrent, and not trip an RCD, and hopefully some possible things
to check in order to locate the fault.

This 'may be' heating associated but does happen when the heating is
off and the boiler is off i.e. could it be associated with hot pipes
near a cable?


I am guessing that you have transposed the terms RCD and MCB here, and what you actually have is a nuisance RCD trip. You will
need to be methodical to track down the cause of this. Some advice is given in the nuisance trip section of this article:

http://wiki.diyfaq.org.uk/index.php?title=RCD


--
Cheers,

John.


I am confused by his post too. If the individual circuits are protected by
RCBOs rather than RCDs it begins to make more sense

--
Graham.

%Profound_observation%


  #4  
Old December 29th 09, 06:23 PM posted to uk.d-i-y
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Posts: 639
Default MCB tripping - why?

Graham. wrote:
"John Rumm" wrote in message o.uk...
nafuk wrote:
Hi, the MCB on my split circuit consumer unit is tripping
sporadically, sometimes when nothing is connected to the downstairs
socket circuit. The MCB protects the RCD protected circuits (upstairs
and downstairs sockets, cooker, hot tank, kitchen sockets). The hot
tank and cooker circuits are turned off at the rcd and are not
connected used/connected yet.The light (non-RCD'd) circuits stay on
when the RCD trips.

Please can someone tell me what causes an MCB to trip, other than
overcurrent, and not trip an RCD, and hopefully some possible things
to check in order to locate the fault.

This 'may be' heating associated but does happen when the heating is
off and the boiler is off i.e. could it be associated with hot pipes
near a cable?

I am guessing that you have transposed the terms RCD and MCB here, and what you actually have is a nuisance RCD trip. You will
need to be methodical to track down the cause of this. Some advice is given in the nuisance trip section of this article:

http://wiki.diyfaq.org.uk/index.php?title=RCD


--
Cheers,

John.


I am confused by his post too. If the individual circuits are protected by
RCBOs rather than RCDs it begins to make more sense

i think an rcd/rcbo can trip even if the circuit is turned off,
if its a singlepole switch
and only cutting power to the live wire,
because a leak from the neutral to earth would trip the rcd
as neutral and live would then be unbalanced.

so maybe disconnect some neuatralsl and see if that stops it.

[g]
  #5  
Old December 29th 09, 06:24 PM posted to uk.d-i-y
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Posts: 639
Default MCB tripping - why?



Overcurrent is the only thing that causes an MCB to trip. A difference in
current between live and neutral causes an RCD to trip. Thus a fault that
causes excess current to flow (slightly excess for a long period or gross
excess for a short period) trips an MCB (or other over-current device
such as a fuse). A few tens of milliamps leakage to earth trips an RCD -
even touching neutral and earth together on a circuit switched off at the
MCB or other single-pole switch in the circuit can cause enough earth-
loop current to flow to do it.



thats waht i was trying to say before i read this!
[g]
  #6  
Old December 29th 09, 11:40 PM posted to uk.d-i-y
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Posts: 117
Default MCB tripping - why?

Sorry, I do have my MCB and RCD's mixed up.

Thank you for the info and pointers..
  #7  
Old December 30th 09, 11:16 AM posted to uk.d-i-y
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Posts: 166
Default MCB tripping - why?

I recently had the same problem and got some excellent advice from
this forum, see my post:

http://groups.google.co.uk/group/uk....54f4f84f46148f

Now fixed.

Brendan

 




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